In the commonly assigned copending application Ser. No. 08/014,543 filed 8 Feb. 1993 (now U.S. Pat. No. 5,402,439) and the corresponding German patent DE 42 04 769 C1, there is described a system for disposing of dust-form substances from combustion plants, including garbage incinerators. In that system, a low-shaft electric furnace can be used.
For the purpose of the present invention, the term "simultaneous" is intended to mean the introduction at the same time, although with separate feed devices, of the meltable dust and the pieces of the grate slag. The simultaneous introduction can be effected at the same place although the materials are separately introduced.
Furthermore, a "closed low-shaft electric furnace" is intended to refer to the type of furnace described in the aforementioned application and patent, such furnaces being utilized also for the recovery of metals from low-quality ores, i.e. for slag-rich smelting processes. A furnace of this type can be readily adapted to the process of the invention for the simultaneous smelting of meltable dusts and pieces of grate slag.
The term "meltable dusts" or the equivalent term "smeltable dust" is intended to refer to dusts which can include fly ash and other particulates of the particle size of fly ash and which normally are entrained in the flue gases of a combustion installation. Such dusts are generally recovered from the flue gas by flue gas cleaning systems. The term refers to dusts which are considered to be metallurgical products as well as dusts containing coke, clarifier sludge, combustion ash and similar substances. The dusts from garbage incinerators and like combustion systems have typical compositions which are recognized in the art.
The term "grate slag" is intended to refer to slags which are recovered from the grates of garbage incinerators or other combustion systems together with the ash which likewise forms. Typical compositions of grate slags of the type produced in garbage incinerators are also known. The grate slags, like ash, frequently have been disposed of in landfills.
In the aforementioned German Patent DE 42 04 769 and the corresponding U.S. application Ser. No. 08/014,543, there is described a process for eliminating dust-like substances from garbage incinerators by a melting process. In this process, the melting is carried out in a closed low-shaft electric furnace having at least one electrode, a bath of molten slag melt being maintained in this furnace by supplying electrical energy thereto.
The dust-like substances are introduced by an immersion lance, i.e. a lance which extends into the melt and terminates therein, all with the aid of a hollow electrode whose outlet end is immersed in the bath. The meltable components of the dust-like substance are bound in the slag. The slag with such bound components is continuously tapped off or discharged from time to time. The waste gases which are produced are subjected to a waste gas cleaning.
In this process, the bath is covered with a coke layer and the dust-like substances are introduced below the coke layer into the molten slag itself.
The melting is carried out under reducing conditions which are established in the slag.
This process has been found to be highly successful. The dust-like substances can include grate ash and, where desired, even grate slag as defined above except that, where the grate slag is used, it must first be milled to a very fine dust-like particle size.
The milling of the grate slag is expensive and requires considerable energy. If the dust-like substances and the milled grate slag are supplied together, they must be intimately mixed which poses problems because the amount of the grate slag generated in a typical garbage incinerator can be ten times greater than the amount of fly ash produced by the combustion and thus, for complete disposal of both waste products by melting, it is necessary to invest heavily in the milling of the grate slag.